One defect which occurs in the manufacture of glass containers is known as a "birdswing". This defect is a results of the two sides of the container contacting each other during the formation of the parison and prior to the blowing of the container into its final shape. The touching of the two sides of the container causes the hot glass to fuse together. As the container is blown to its final shape its sides that have touched will move away from each other drawing a small thread of glass therebetween, thus forming what is termed a birdswing. A birdswing need not consist of a complete thread running between the two inner walls of the container, as the thin filament of glass which would span the diameter of the container may break due to its becoming extremely thin and thereby cooling very rapidly, to the extent that it would break rather than stretch. However in every instance when a birdswing is formed, there will be small conical protruberances from the sidewalls in a direction of the opposite sidewalls, coincident with the formation of the birdswing.
One type of birdswing detector is disclosed in U.S. Pat. No. 3,438,492, issued to W. R. Albers on Apr. 15, 1969. A second system is disclosed in U.S. Pat. No. 3,662,883, issued to J. R. Sager on May 16, 1972. Both of these systems operate by passing a light beam through the wall of the container in order to detect the presence of a birdswing. Since other defects, such as checks, cracks or blisters might appear in the wall of the container, the accuracy of the above named systems may be impaired by these other types of defects being mistaken for birdswings. The present invention avoids this potential problem by passing a light beam which is used to inspect a container through the opening of the finish of the container rather than through the container wall. The light beam will thus strike a birdswing, if present, before passing through any other portion of the container.